Production of fluosilicic acid

ABSTRACT

An improved process for the recovery of fluosilicic acid from recycle streams in the manufacture of phosphoric acid by the sulfuric acid digestion of phosphate ore.

United States Patent [1 1 Darwin et al.

[ 1 Oct. 9, 1973 PRODUCTION OF FLUOSILICIC ACID [73] Assignee: UnitedStates Steel Corporation, Pittsburgh, Pa.

[22] Filed: Jan. 26, 1972 [21] App]. No.: 220,893

[52] US. Cl. 423/321 [51] p C01b 25/16 [58] Field of Search [56]References Cited UNITED STATES PATENTS 3,615,195 10/1971 Bierman et al.423/321 2,933,372 4/1960 Manning 423/321 Primary Examiner-Oscar R.Vertiz Assistant Examiner-Gregory A. Heller Attorney-Arthur J. Greif 5 7ABSTRACT An improved process for the recovery of fluosilicic acid fromrecycle streams in the manufacture of phosphoric acid by the sulfuricacid digestion of phosphate ore.

7 Claims, 6 Drawing Figures PATENTEUUET 91w 3.764.658

SHEET 1 {1F 3 FIG.

Phosphate Ore Wash Liquor H2504 Phosphate Ore C050 Phosphoric DigesterFilter Acid Ste am, Sit-' Vacuum Str/pper- H2 504 Chamber di/uter 1'Si/I'ca Steam Steam,S/'F

70 1 H 50 fjj Silica Recycle Stream Vacuum Str/pperchamber dilute,Pre-dI/uter 'Fram C080 Filter Steam Fluoride 7'0 Steam, Sit-' Digester H80 r-$iliea Recycle Stream Vacuum Stripper chamber dilute, Pre-dI/utergrit/2,6080

Steam PATENTED UN 9 75 SHEET 2 BF 3 FIG. 4.

FLUOR/NE EVALUATION'VERSUS F/S/ (/0 96 DIST/LLAT/OIV A7 8 IN. VACUUM)F/Si Mal Raf/'0 PRODUCTION OF FLUOSILICIC ACID BACKGROUND OF THEINVENTION The wet process for the manufacture of phosphoric acidutilizes as raw materials sulfuric acid and phosphate rock. The mainconstituent of NorthAmerican phosphate ore is the mineral fluorapatite,3Ca (PO 'CaF which has a F:P,O,, ratio of about 0.09. In past years onlythe phosphorus values in the mineral have been utilized, the fluorinebeing considered a contaminant that was not worth recovering. During thelast two decades, however, the strong trend toward high analysisfertilizer compositions, such as triple superphosphate and ammoniumphosphates obtained from wet process phosphoric acid has accelerated theintroduction of fluorine scrubbing systems, partly for pollution controland partly for recovery and sale of fluosilicic acid and fluosilicatesalts.

When the phosphate rock is digested with a strong mineral acid, thefluorine is released and forms hydrofluoric acid. The hydrofluoricacid'thus produced reacts immediately with the silica that is alwaysassociated with the phosphate rock to produce fluosilicic acid accordingto the following reaction.

The H SiF is in solution in-the phosphoric acidand remains in theproduct acid unless special procedures are provided for its removal. Itis also desirable to recover the fluorine present in other streams inthewet process. For example, afterdigestion of the phosphate todecompose the fluosilicic acid and evolve the fluorine in the form ofsilicon tetrafluoride, SiF Morriset al., Canadian Chemistry andMetallurgy, August, 1937, describe one such process. Smaltz et al.in'two U.S.

Pat., Nos. 3,498,746 and 3,498,747, describe a process in which the pondwater or recycle streams (746) from the wet process are contacted withconcentrated sulfuric acid to heat the mixture to about 250F and evolvethe fluorine. The concentrated sulfuric acid is diluted to 25 70% H SO(746) or 40 80% H SO (747). In the 74 6 patent, it is suggested thathigher fluorine evolution may be obtained by sparging steam into thedilution tank. Bierman, Jr. et al., U.S. Pat. No. 3,615,195, offers afurther improvement by preheating the fluorine containing stream and byadding silica to the dilution tank to convert the HF present in thesolution to SiF,.

The known procedures howeverare all deficientin that they provide eitheronly limited recovery of the fluorine values in the recycle streams orsuffer from tie in complications with the operation of the phosphaterock digestion plant. The factors which control both the amount ofrecovery and which lead to tie in complication are (l the sulfuric acidconcentration in the diluter, (2) the heat input to the diluter, and (3)the heat balance of the phosphate plant digestion step.

To achieve a satisfactory level of fluorine evolution, the sulfuric acidconcentration in the acid dilution tank should be'about 30 to 40 weightpercent. However, the quantity of sulfuric acid that may be employed islimited by the requirements of the phosphate rock digestion step. Sincethe quantity of recycle from the phosphoric acid plant is too large inproportion to the available sulfuric acid to achieve the requiredminimum acid strength only a portion of the recycle stream can betreated resulting in only a limited quantity-of the fluorine in thesystem being liberated.

The heat input to the dilution step should be as large as possible asthe fluorine evolution from the mixed acids increases with increasedtemperature. However, the residual liquid, after fluorine evolution,must be at atemperature no higher than 200F before it can be in troducedinto-the phosphate digestion step or the heat balance in the digestorwill-be disturbed andunsatisfactory gypsum crystal formations willresult.

SUMMARY OF THE INVENTION This invention overcomes these problems andprovides an improved process-for the recovery of fluosilicic acid whichallows (l) a more independent operationotthe'fluosilicic acid plant fromthe phosphoric acid plant, (2) provides more heat input at thefluosilicic acid plant without overloading the cooling facilities of'the phosphoric acid plant operation, and (3) achieves substantiallyhigher overall fluorine recovery by improved reaction conditions.

These benefitsare obtainedby using a novel combination of a steamstripping operation conducted at or above atmospheric pressure with alow pressure volatilization step. Modifications of this basic concept,involving adjustment of the fluorine/silica ratio in the strippingvessel, preheating of the input streams, premixing. of the input streamsand maintenance of a speci fled sulfuric acid concentration, all serveto increase even further the benefits obtained.

In the process, recycle streams from the sulfuric acid digestion ofphosphate ore are mixed with concentrated sulfuric acid in a stripping(dilution) vessel. The heat of dilution of the acid generates in situ.The generated steam acts as a transport medium for the silicontetrafluoride and hydrogen fluoride present in the recycle streams.Additional steam is added to the vessel to increase the quantity ofsilicon tetrafluoride and hydrogen fluoride removed. An acid solublesilica compound may be added to form additional silicon tetrafluoride.

The stripped liquor is then introduced into a vacuum chamber where thereduced pressure releases more steam, which in turn carries awayresidual SiF and hydrogen fluoride. At the same time the temperature ofthe liquor is lowered, thus maintaining the heat balance in thephosphate digestor. In one embodiment of the invention, a portion of therecycle stream is injected directly into the vacuum chamber to provideeven more fluorine evolution.

DETAILED DESCRIPTION Essentially, the process of the present inventioncomprises four basic steps which provide for the efficient, economicalremoval and recovery of fluoride values from recycle streams in the wetprocess for the production of phosphoric acid.

In the first step a portion of the recycle stream is mixed withconcentrated sulfuric acid in the stripper- .diluter to provide asulfuric acid concentration in the resulting mixed acids in the range of20 70 weight percent, preferably 25 35 weight percent. That portion ofthe recycle stream not utilized in this step is fed directly into thevacuum chamber at step 2. The heat of dilution of the sulfuric acidproduces steam which acts as a transport medium to carry away vaporizedSiF and hydrogen fluoride. Additional steam may be sparged into thestripper-diluter to carry away additional SiF, and hydrogen fluoride.Preferably, sufficient quantities of an silica are added to the mixtureto provide a fluorinezsilica molar ratio of from 5 to l to 6 to 1. It isalso possible to add impure fluorides or fluosilicates to the mixture ofacids to increase the fluorine evolution. Examples of such materialsinclude potassium, sodium and calcium fluosilicate and by-productsilicofluorides, such as phosphoric acid reactor scale and solidsproduced in the concentration of dilute wet process phosphoric acid.Other fluoride materials and ores such as sodium, potassium, calcium ormagnesium fluoride and impure fluoride containing acids such ashydrofluoric or fluosilicic may also be added.

in the second step the partly defluorinated liquor from thestripper-diluter is introduced into a vacuum chamber to further evolvefluorine containing compounds and to coolthe liquor. In the third stepthe cooled liquor from the vacuum chamber is recycled to the phosphaterock digestion step. The fourth step consists of recovering the evolvedfluorine compounds in the evolved vapors by conventional means.

The invention is most easily described by reference to the drawings.P16. 1 shows the basic embodiment of the invention. A portion of thephosphoric acid containing liquids from the calcium sulfate filter ismixed with concentrated sulfuric acid in the stripper-diluter. Steam maybe sparged into the liquor. In conjunction with the in situ generatedsteam, it strips some SiFi from the liquor. Silica may be added asnecessary to maintain the proper fluorine-silica ratio. The fluorinedepleted liquor is sent to a vacuum chamber where steam is generated dueto the lower pressure and the vapors containing residual SiF, arecollected.

FIG. 2 shows a variation on the recovery procedure. A pre-diluter isplaced in the system and the silica and a minor portion (about 10percent) of the concentrated sulfuric acid are therein mixed with therecycle stream. The advantage of the pre-dilution step is that residualcalcium in the recycle stream is precipitated and the subsequentcompletion of the dilution is more effectively carried out.

FIG. 3 shows a preferred embodiment of the recovery procedure. A portionof the recyclestream from the calcium sulfate filter is injecteddirectly into the vacuum chamber by-passing the stripper-diluter. Thisserves to supply more fluorine to the vacuum chamber to obtain thebenefit of the steam generated therein.

Pond water and recycle streams in the phosphoric acidplant contain thefluorine predominantly inthe form of fluosilicic acid. A smallerportion-is present as hydrogen fluoride, fluorophosphoric acid, andfluorine complexes of iron and aluminum. The volatilization of thefluorine is most readily accomplished as silicon tetrafluoride. It istherefore advantageous to provide additional silica and/or fluoride ifthe streams do contain not enough reactive silica in proportion to thefluorine content. An optimal utilization of silica is accomplished at afluorine-silica molar ratio of about 5 to l to 6 to l as is evident fromFIG. 4, although either higher or lower ratios are also suitable. Theaddition of silica and/or fluoride is most conveniently achieved at thepre-dilution tank as shown in FIGS. 2 and 3.

Formation of silicon tetrafluoride in the process streams occurspredominantly by decomposition of fluosilicic acid and by reaction ofsilica or silica derivatives with hydrogen fluoride.

H SiF I SiF ZHF SiO, 4HF SiF 2H O The reactions are acid catalyzedequilibrium systems. The concentration of silicon tetrafluoride in theliquid and its vapor pressure at ordinary process conditions is verylow.

Consequently, the volatilization of fluorine is greatly improved byconditions which provide a transport medium to remove the silicontetrafluoride from the liquid, and which accelerate the rate of silicontetrafluoride formation. Such improvement is achieved by applying asteam sparge which accelerates the silicon tetrafluoride removal, and ahigh acidity which allows a rapid response of the equilibrium to producemore silicon tetrafluoride as it is removed from the system. FIG. 5demonstrates the effect of steam formation (boiling) on thevolatilization of fluorine.

Since steam sparging introduces additional water its application islimited to an amount equal to 0 l5 weight percent, preferably 5 10weight percent, of the recycle acid. Steam quantities up to 25 weightpercent or more may be used but will decrease the fluorine concentrationin the vapor thereby lowering the efficiency of the fluosilicic acidrecovery system.

The fluorine volatilization increases as the temperature is raised.However, the maximum temperature is limited by equipment considerations.Thus, although the stripper-diluter may be operated at or aboveatmospheric pressure and at a temperature in the range of 180 350F, thetypical operating conditions are atmospheric pressure and a temperaturein the range of 230 250F.

The sulfuric acid concentration in the mixed acids is chosen byreference to the overall fluorine recovery. The feed sulfuric acid mayhave a concentration in the range of 50 percent to fuming, preferably 93to 99 percent. Highest volatilization is obtained in sulfuric acidconcentrations in the stripper-diluter of about 40% H 80 However, thisrequires the by-pass of a substantial portion of the recycle stream andtherefore lowers the quantity of the fluorine containing stream whichcan be treated. in the present invention, the stripperdiluter may beoperated with H S O, concentrations of from 20 to percent, althoughoptimum fluorine yields are achieved'by selecting to operate at thelowest satisfactory acid concentration (25 35% H and to compensate forthe lower heat of dilution by additional heat input such as recyclepre-heating and by flashing the residual liquor in the vacuum chamber.Thus, for the first time a process is provided which treats the entiretyof the recycle streams from the phosphoric'acid plant.

The sulfuric acid feed stream should be at a temperature in'the range offrom 50F to its boiling temperature.'Preferably, it should be atemperature in the range of from to 200F. it may be injected underpressure. The recycle stream from the phosphoric acid plant should be ata temperature in the range of from 100 to 250F, the higher temperaturebeing used when the stream is pressurized. Preferably, the recyclestream temperature should be in the range of from 210 to 225F.

The silica source added to the mixed acid is an amorphous silicaselected from such compounds as diatomaceous earth, silicic acid,silica, and silicic acid salts.

The stripper-diluter should be run at temperature of from 180 to 350F,preferably at a temperature of from about 230 to 250F. The pressure inthe vacuum chamber may vary widely. Good results are obtained at to 25inches Hg, with optimum performance at to inches Hg. The temperature ofthe feed streams to the vacuum chamber should be from 170 to 240F,preferably from 185 to 205F and the temperature of the liquid exitingfrom the vacuum chamber should be from 180 to 200F, preferably from 185to 195F.

In one embodiment of the invention from to 75 percent of the phosphoricacid recycle stream is added to the stripper-diluter and the remainderdirectly to the vacuum chamber. Preferably, from 50 to 65 percent of therecycle stream is added to the stripper-diluter.

The invention is further illustrated by the following non-limitingexamples.

Example I Recycle acid (100.0 g.) containing 20.41% P 0 2.44% SOf, 1.91%F, and 0.69% SiO was mixed with 98% H 80 (50.0 g.) and heated in aplastic vessel under reduced pressure (8 in. vacuum) until 10 percent byweight of the recycle acid was volatilized. The fluorine to silica moleratio was varied by addition of dry silica over a range from 3.4 to11.1. The following results (FIG. 4) were obtained:

F/Si0 Mole Ratio: 3.4 5.4 6.2 7.0 11.1

F Evolution: 63 63 58 53 41 As evident, optimum silica utilizationoccurs at 5-6 F/Si.

Example 11 A mixture of recycle acid and H SO as described in Example 1was adjusted with dry silica to a 5.4 F/SiO mole ratio, heated toboiling at 8 in. vacuum, and the volatile material condensed in anice-cooled trap. The procedure was repeated with different heat inputsto increase the amount of steam formation by boiling, while maintaininga constant distillation time of approximately 20 min. The collectedcondensate gave the following accumulative results (FIG. 5):

% Distillation: 6.4 10.2 14.5 19.0

% F Evolution: 52 63 73 76 Weight of condensate expressed as percent ofrecycle acid. The data indicate that the F evolution is dependent onsteam formation, producing more volatile F with increasing distillation.

Example III A mixture of recycle acid and H SO as described in ExampleII was heated to boiling at a rate to achieve approximately 10 percentdistillation over a 20-min. period. The procedure was repeated atdifferent pressures to vary the reaction temperature. The results,listed below, demonstrate the higher fluorine volatilization at highertemperatures.

Vacuum, in. Hg: 8.5 13.0 16.0 22.5 Temp., "F: 224214 202 194 FEvolution: 63 52 46 44 Example IV Recycle acid (387.1 g.) containing24.1% P 0 1.88% S0 1.83% F, and 0.52% SiO was adjusted withsilica (1.46g.) to a F/Si mole ratio of 5.4 and heated to 160F. The resulting slurrywas fed by gravity into a Teflon reactor together with 98% H SO (193.0g.) maintained at 100F. The hot mixture was stirred under adiabaticconditions for 50 min., allowing volatile material to vent atatmospheric pressure. Subsequent flashing of the hot liquid into avacuum chamber at 15 in. vacuum under adiabatic conditions reduced thefluorine concentration to 0.43% F. The weight loss due to volatilizationcorresponded to 6.9% of the recycle acid. The F evolution based on thechange in concentration was 67.8 percent.

We claim:

1. In a process for recovery of fluorine containing compounds from aphosphate ore having fluoride impurities associated therewith whereinthe phosphate ore is digested with sulfuric acid to produce phosphoricacid and precipitated calcium salts, the calcium salts are filtered andthe filter cake washed with an aqueous washing liquor to remove adheringphosphoric acid, the resulting phosphoric acid containing aqueouswashing liquor is' contacted with sulfuric acid to evolve steam and themajor portion of the fluorine values, and the acidified,fluorine-depleted washing liquor returned to the digestion step, theimprovement comprising:

a. contacting the washing liquor with sufficient concentrated sulfuricacid to elevate the sulfuric acid concentration of said liquor to avalue of from 20 e. recycling the liquor from step c to the phosphate Iore digestion step.

2. The process of claim 1 wherein from 0 to 15 weight percent steam isadded to the mixture in step a.

3. The process of claim 1 wherein from 5 to 10 weight percent steam isadded and the sulfuric acid concentration is elevated to a value of from25 to 35 weight percent.

4. The process of claim 1 wherein sufficient reactive silica is added instep a to adjust the fluorinezsilica molar ratio to from 5 to l to 6 to1.

5. The process of claim 1 wherein a fluoride containing compound isadded to the mixture of step a.

6. The process of claim 1 wherein from 30 to 75 percent of the washingliquor is contacted with the sulfuric acid instep a and the remainingportion of the washing liquor is introduced directly into the vacuumchamber in step c.

7. The process of claim 6 wherein from 50 to percent of the washingliquor is contacted with the sulfuric acid in step a and the remainingportion of the washing liquor is introduced directly into the vacuumchamber in step c.

2. The process of claim 1 wherein from 0 to 15 weight percent steam isadded to the mixture in step a.
 3. The process of claim 1 wherein from 5to 10 weight percent steam is added and the sulfuric acid concentrationis elevated to a value of from 25 to 35 weight percent.
 4. The processof claim 1 wherein sufficient reactive silica is added in step a toadjust the fluorine:silica molar ratio to from 5 to 1 to 6 to
 1. 5. Theprocess of claim 1 wherein a fluoride containing compound is added tothe mixture of step a.
 6. The process of claim 1 wherein from 30 to 75percent of the washing liquor is contacted with the sulfuric acid instep a and the remaining portion of the washing liquor is introduceddirectly into the vacuum chamber in step c.
 7. The process of claim 6wherein from 50 to 65 percent of the washing liquor is contacted withthe sulfuric acid in step a and the remaining portion of the washingliquor is introduced directly into the vacuum chamber in step c.